1. Field
The subject matter disclosed herein relates to ultrasonic imaging systems.
2. Information
“Imaging” refers to a process of capturing visual features of one or more objects of interest. “Ultrasonic imaging” refers to a process of imaging which comprises the processing of acoustic signals. Medical professionals using ultrasonic imaging technology typically employ images of sufficient resolution and clarity for proper diagnosis. An ultrasonic image is formed from transmitting an ultrasonic waveform through tissue and processing resulting reflections and/or transmissions from an object of interest. While using higher frequency transmit waveforms may improve image resolution, the higher frequency waveforms typically introduce increased system noise that may degrade image clarity. System noise also becomes of increasing concern when imaging objects are located at significant depths in tissue (e.g., a distance between a surface of a tissue body and an object of interest within the tissue body). Accordingly, for imaging deep objects, a trade off between resolution quality and image clarity may be made by adjusting the ultrasonic waveform frequency. An ultrasound transducer typically transmits and receives ultrasonic waveforms within a predefined frequency range (e.g., 1.0 to 4.0 MHz) and carrier frequency (e.g., 7.0 to 15 MHz). An operator of an ultrasonic imaging system typically manually adjusts the ultrasonic waveform frequency.
In addition to system noise, the effects of ultrasonic “clutter” may also degrade image quality. Clutter is typically the result of ambient reflections from inert tissue or objects that are not of interest. Typically, bright off-axis targets introduce off-axis scattering that may hinder the ability of commercial ultrasound systems to image desired targets. Echoes from these off-axis targets typically generate broad clutter which may overshadow the signal from the targets, greatly reducing image contrast and/or axial resolution. For example, the rib cage, in cardiac imaging, and the bladder, in abdomen imaging, may result in this off-axis clutter.
An ultrasound imaging system typically operates in a “fundamental” mode in which an ultrasound transducer transmits an ultrasonic waveform at a first carrier frequency and/or bandwidth, and receives and processes reflections at the first carrier frequency and/or bandwidth. An ultrasound imaging system is also typically selectable to operate in a “harmonic” mode in which an ultrasound transducer transmits an ultrasonic waveform at a first carrier frequency and/or bandwidth, and receives and processes reflections at a second, and higher carrier frequency (e.g., the second carrier frequency being an integer multiple of the first carrier frequency such as about twice the first carrier frequency). As clutter may be present predominately at lower frequencies at about the first carrier frequency, by receiving harmonic components of the reflected signal at the higher frequencies (and rejecting lower frequencies of the reflected signal) using a harmonic mode may improve clutter rejection performance. However, the received harmonic signals are typically not as strong as the signals received at the first carrier frequency. Therefore, using a harmonic mode may degrade signal-to-noise performance.